The present invention relates to a voltage step-up circuit for generating a high voltage which is used to change the threshold voltage of an electrically erasable, non-volatile semiconductor memory.
The electrically erasable, non-volatile memories include an electrically erasable programmable read only memory (EEPROM) and an electrically alterable read only memory (EAROM). To alter the contents of an EEPROM, for example, it is necessary to apply a high voltage to the memory cells of the EEPROM to change the threshold voltage of the memory cells. The high voltage usually ranges from 15 to 20 volts. To generate this high voltage, a voltage step-up circuit is used. One example of the voltage step-up circuit for a non-volatile memory is shown in FIGS. 3-8.
In FIG. 3, a voltage step-up circuit 3 is connected to an EEPROM 1 via a decoder 2 which designates a series of addresses in the EEPROM 1. The EEPROM 1 is composed of a number of memory cells 1a, one of which is shown in FIG. 8. The memory cell is consists of a pair of source drains 18 formed on a substrate 17, a floating gate 16, and a control gate 15.
The voltage step-up circuit 3 includes an oscillator 10 for generating a high speed clock signal, a first series 20 of three inverters 20a, 20b, 20c for inverting the clock signal, a second series 21 of two inverters 20b, 20cand a series 30 of MOS transistors 30a in cascade connection, the gates of which are alternatingly connected to the first and second inverter series 20 and 21 via a capacitor 40. As shown in FIG. 5, each of the inverters 20a, 20b, 20c is composed of a p-channel transistor 200 which is connected to a high-potential power line and turned on upon reception of a clock pulse g, and an n-channel transistor 201 which is connected to a low-potential power line.
In operation, the phase of a clock signal g from the oscillator 10 is inverted by the first and second inverter series 20 and 21. The phase-inverted, high-speed clock signal is inputted to the transistor series 30 to alternatingly turn on the transistors 30a. Consequently, the voltage applied to the gate of a transistor 30a is increased by the transistor 30a on the preceding stage. As shown in FIG. 4, the voltage applied to the drain of a transistor 30a is stepped up according to the bootstrap principle. To accomplish this, a high-speed clock pulse signal of a few MHz is required. The high voltage from the step-up circuit 3 is applied to a designated address series of the EEPROM 1 via the decoder 2 for writing new data.
As shown in FIG. 6, in the period (t.sub.1 -t.sub.2) when the output e of the inverter 20b, which is rising from a low level to a high level in response to a clock pulse g, is lower than the threshold voltage V.sub.TH of an n-channel transistor, only the p-channel transistor 200c is on so that the output f is at a high level. In the period (t.sub.2 -t.sub.3) when the output e is higher than the threshold voltage V.sub.TH of an n-channel transistor but lower than the threshold voltage V.sub.TH of a p-channel transistor, both the p-channel and n-channel transistors 200c and 201c are on so that the output f is at a fraction of the line voltage which is proportional to the On-time resistance of the n-channel transistor over the On-time resistance of the p-channel and n-channel transistors. In the period (t.sub.3 -t.sub.4) when the output e is higher than the threshold voltage V.sub.TH of a p-channel transistor, the p-channel transistor 200c is off so that the output f is at a low level.
When the output e is drops from the high level to the low level, the events take place in the order opposite to those described above.
In the above inverter, a large amount of through current flows in the period when both the p-channel and n-channel transistors 200c and 201c are on. In order to prevent this through current, Japanese Patent Application Kokai No. 63-38250 has proposed, as shown in FIG. 7, that a resister 202 is connected between the drains of p-channel and n-channel transistors 200b and 201b of a preceding inverter 20b, and that the gates of p-channel and n-channel transistors 200c and 201c of the subsequent inverter 20c are connected across the resister 202, thereby shortening the period when both the p-channel and n-channel transistors 200c and 201c are on, thus minimizing the through current.
A burst of through current in the inverters 20a-20c of the step-up circuit 3 produces electromagnetic waves, which are absorbed by the memory cells to produce electron-hole pairs or free carriers in a space between the floating gate and the substrate and other areas where carriers are exchanged. Consequently, at the time of writing when a large amount of through current flows, the change of a threshold value, which is determined by the tunnel effect, can be disturbed. The smaller the cell area per memory transistor or the larger the memory capacity, the larger this effect. In addition, the through current produces noise.